1. Field of the Invention
The present invention relates to a wiring board used for an electronic device, and in particular to a wiring board with a built-in electronic component and a method for producing the same.
2. Description of the Related Art
FIG. 15 is a cross-sectional view of a general printing wiring board 110 provided with an electronic component 104.
The printing circuit board 110 includes a support member 101 formed of an insulating resin and a conductive pattern 102. The conductive pattern 102 is produced by forming a conductive layer made of, for example, copper foil on the support member 101 and then etching the conductive layer. The printing circuit board 110 includes a solder resist layer 103 provided on the support member 101 and on the conductive pattern 102. On the conductive pattern 102, the solder resist 103 is provided except for areas on which the solder resist 103 is not necessary for the purpose of connection with the electronic component 104 or electric or physical connection with other components, or for other purposes. The electronic component 104 is mounted on the solder resist layer 103. A terminal 105 is provided on a side surface of the electronic component 104. A part of the terminal 105 is an electronic component-mounting land (i.e., a part of the conductive pattern 102), and the terminal 105 and the electronic component-mounting land are connected to each other via a solder member 106.
In the example shown in FIG. 15, only one conductive layer is provided on the support member 101 such that the conductive pattern 102 is provided on only one surface of the support 101 for the simplicity of explanation. A multi-layer printing wiring board including a multi-layer conductive pattern formed of a plurality of conductive layers is in wide use.
A wiring board with a built-in electronic component is produced by the following two methods based on the structure shown in FIG. 15 including the printing wiring board 110 and the electronic component 104. (1) A spacer is bonded to an area of the printing wiring board 110 where the electronic component 104 is not provided, and a resin plate is bonded to the spacer so as to cover the electronic component 104. (2) The printing wiring board 110 and the electronic component 104 are molded together with a resin.
FIG. 16 is a cross-sectional view of a wiring board with a built-in electronic component which is disclosed in Japanese Laid-Open Publication No. 2001-53447. The wiring board with a built-in electronic component includes a support member 201 formed of an insulating resin or a multi-layer printing wiring board and an accommodating section 202 for accommodating an electronic component 104 in the support member 201. The accommodating section 202 is provided in an upper portion of the support member 201 or in the entirety of the thickness direction of the support member 201. The electronic component 104 is accommodated in the accommodating section 202. A top surface of the electronic component 104 is on the same level as a top surface of the support member 201. An insulating resin layer 203 is provided on a surface of the electronic component 104, and a wiring pattern 204 is provided on the insulating resin layer 203. A terminal 105 of the electronic component 104 and the wiring pattern 204 are electronically connected to each other via a through-hole provided in the insulating resin layer 203. The through-hole is formed by the same method as a laser method which is used for producing a build-up wiring plate.
FIG. 17 is a cross-sectional view of another exemplary wiring board with a built-in electronic component. The wiring board with a built-in electronic component includes a printing wiring board 110 including a support member 101, and a conductive pattern 102 and a solder resist layer 103 provided on the support member 101 as in the example shown in FIG. 15. The wiring board with a built-in electronic component shown in FIG. 17 further includes an electronic component 301 provided on the printing wiring board 110 by printing. The electronic component 301 is formed of, for example, a conductive paste or a dielectric paste, and is provided on the conductive pattern 102 by printing. The electronic component 301 is covered with an insulating resin layer 302 printed on the entire surface of the printing wiring board 110. The insulating resin layer 302 covers the electronic component 301 and also prevents shortcircuiting between the electronic component 301 and other elements.
FIG. 18 is a cross-sectional view of a wiring board with a built-in electronic component, in which the wiring board has a heat dissipation function. The wiring board with a built-in electronic component includes a metal core substrate 401. Both a top surface and a bottom surface of the metal core substrate 401 are covered with an insulating resin layer 402. On the insulating resin layer 402 on both surfaces of the metal core substrate 401, an electronic component-mounting land as a part of a conductive pattern 102 and a solder resist layer 103 are provided. An electronic component 104 is provided on the solder resist layer 103 on the top surface of the metal core substrate 401. A terminal of the electronic component 104 is connected to the electronic component-mounting land via a solder member 106. The conductive pattern 102 on the top surface of the metal core substrate 401 and the conductive pattern 102 on the bottom surface of the metal core substrate 401 are electrically connected to each other via a through-hole 403.
The electronic component 104 mounted on the metal core substrate 401 generates a large amount of heat. The metal core substrate 401 is used in the case where, for example, (1) the heat of the electronic component 401 is not dissipated sufficiently quickly by a usual method, (2) there is no space for accommodating a large heat sink, and (3) the rigidity of a resin substrate is not sufficient for carrying a heavy electronic component that is to be mounted.
The above-described conventional devices have the following problems.
In the wiring board with a built-in electronic component shown in FIG. 15, the electronic component 104 mounted on the general printing wiring board 110 is covered with a resin plate, or molded with a resin or the like together with the printing wiring board 110. When a great number of electronic components 104 are mounted at high density with this structure, there is a possibility that no space results for the provision of a spacer. In the structure obtained by molding with a resin or the like, the heat dissipation efficiency from the electronic component 104 is possibly deteriorated. Accordingly, the structure obtained by covering the electronic device with a resin plate or molding the electronic device is used, for example, for improving the ease of handling the wiring board, for forming the circuit as a black box, or for increasing the shock resistance.
This structure with a plurality of electronic components is obtained by mounting an additional electronic component on the printing wiring board 110 which already has an electronic component 104, or by electrically connecting the electronic components 104 mounted on the printing wiring board 110 to other elements and the like. In such a stage of production, soldering is performed. Solder reflow or the like caused by soldering re-heats and thus re-melts the solder member 106, for solder-connecting the terminal of the electronic device 104 to the electronic component-mounting land. As a result, the electronic device 104 may possibly come off from the printing wiring board 110. In addition, the solder used here, which is formed of a heavy metal or the like, is hazardous to the environment.
In the case of the wiring board with a built-in electronic component shown in FIG. 16, it is necessary to size the accommodating section 202 precisely in accordance with the size of the electronic component 104, which is quite difficult. Especially in the case where a printing wiring board is used as the support member 201, the accommodating section 202 is not formed by molding or the like. Therefore, the accommodating section 202 needs to be formed in each printing wiring board. Usually, several tens or several hundreds of electronic components 104 are mounted in one wiring board. It is practically impossible to form the accommodating sections 202 for all of the electronic components 104.
The electronic components 104 have different heights depending on the type, and even the same type of electronic components 104 are uneven in size. It is very difficult to form the accommodating sections 202 such that the top surfaces of all the electronic components 104 are at the same level as the top surface of the support member 201.
In the case where a printing wiring board is used as the support member 201, the accommodating section 202 formed in accordance with the size of the electronic component 104 prevents formation of the conductive pattern 204 at the location of the accommodating section 202. Considering that the insulating capability of the insulating resin layer 203 is lowered in the vicinity of the accommodating section 202, the conductive pattern 204 needs to be distanced to some extent from the accommodating section 202. In this case, the density of the conductive pattern 204 is significantly lowered. Today, electronic components are often designed to be mounted at an interval of several millimeters or less. With such a design, it is difficult to form the accommodating sections 202, much less to form the conductive pattern 204 between the accommodating sections 202. The insulating resin layer 203 is removed using laser light, drilling or the like. Such removal needs to be performed with a high level of control in order to prevent damaging the terminal 105 of the electronic component 104.
In the case where a printing wiring board is used as the support member 201, the thickness of the electronic component 104 is usually greater than the thickness of the printing wiring board. Therefore, the electronic component 104 actually passes through the accommodating section 202 in the printing wiring board, rather than being built in the printing wiring board. The electronic component 104 in this state is not sufficiently stable in terms of electronic connection or mounting state.
Accordingly, the structure shown in FIG. 16 is actually used for specific purposes, for example, for lowering the projecting height of the electronic component mounted on the printing wiring board. This is performed with the sacrifice of lower wiring density and the cost for producing a special-shape element.
The structure shown in FIG. 17 has the advantages that incorporation of the electronic component 301 does not significantly increase the total thickness of the wiring board and even re-heating by solder reflow or the like does not cause the electronic component 301 to come off. However, the electronic component 301 which can be mounted is limited to a printable component. Accordingly, the structure shown in FIG. 17 is actually usable only for, for example, a resistor formed of carbon or a capacitor having a very small capacitance. Production of the electronic components 301 by printing a paste is not practical for the following reasons. Even with the same designed size and thickness, the electronic components 301 actually produced are quite dispersed in size and thickness. Moreover, the heat used for soldering the electronic components 301 further varies and thus further disperses the size and thickness thereof.
With the structure shown in FIG. 18 in which the metal core substrate 401 is used as the support, the entire surface of the metal core substrate 401 and the inner wall of the through-hole 403 need to be completely covered with the insulating resin layer 402 because the metal core substrate 401 is conductive. However, it is difficult to completely cover the inner wall of the through-hole 403. It is also difficult with this structure to form small-diameter through-holes and provide a multi-layer signal wiring layer in order to increase the wiring density. Even when the multi-layer signal wiring layer is provided, the layer is provided between the electronic component 104 and the metal core substrate 401, which significantly spoils the heat dissipation efficiency.